Surface mount component having magnetic layer thereon and method of forming same

ABSTRACT

A microelectronic assembly, a surface mount component and a method of providing the surface mount component. The assembly comprises: a substrate having bonding pads disposed on a mounting surface thereof, the bonding pads including a ferromagnetic material therein; solidified solder disposed on the bonding pads; and a surface mount component bonded to the substrate by way of the solidified solder and including a magnetic layer disposed on a substrate side thereof, the magnetic layer being adapted to cooperate with the ferromagnetic material in the bonding pads to establish a magnetic force of a sufficient magnitude to hold the surface mount component on the substrate before and during soldering.

FIELD

Embodiments of the present invention relate to electronic assembliesand, more particularly, to anti-flip/anti-shift/anti-tombstoningstructures and associated fabrication methods.

BACKGROUND

One of the conventional ways of mounting components on a substrate iscalled surface mount technology (SMT). SMT components have terminals orleads (generally referred to as “electrical contacts”, “bumps”, or“pads”) that are soldered directly to the surface of a substrate. SMTcomponents are widely used because of their compact size and simplicityof mounting. The electrical contacts of an SMT component are coupled tocorresponding electrically conductive mounting or bonding pads (alsoreferred to as “lands”) on the surface of the substrate, in order toestablish secure physical and electrical connections between thecomponent and the substrate. In order to fabricate PCBs in higherdensities, it is known to surface-mount certain small passivecomponents, such as capacitors, resistors, and inductors. The resultingelectronic system can be manufactured at a lower cost and in a morecompact size, and it is therefore more commercially attractive.

Before SMT components are mounted on a substrate, the substrate pads areselectively coated with corresponding solder deposits. Next, thecomponent is carefully positioned or “registered” over the substrate, sothat its electrical terminals are aligned with the correspondingsubstrate pads. Finally, in an operation known as “soldering,” thecomponent terminals and the PCB pads are electrically and mechanicallybonded together through a solidification of the solder deposits. Anexample of a soldering method includes solder reflow, a process duringwhich the component terminals and the PCB pads are first heated to atemperature that melts the solder deposit, and during which thecombination is then allowed to cool, so that the solider solidifies intosolidified solder, and such that the terminals and pads thus make properelectrical and physical connections.

Typically, for example as seen in FIGS. 1 a and 1 b, a substrate 10 haspairs of pads 12 to which terminals 14 of SMT components, such as dieside capacitor or DSC 16, can be mounted. Solder resist 15 is disposedbetween the two pads 12. Asymmetrical, lateral, surface-tension forcesdue to uneven surface tension of solder deposits 22 on the pads 12during soldering can cause the DSC 16 to either shift, as seen in FIG. 1a, or tombstone, as seen in FIG. 1 b. FIG. 1 a shows a top view of DSC16 as having shifted away from one of the substrate pads 12 to cover anadjacent substrate pad, while FIG. 1 b shows a side view of DSC 16 ashaving tombstoned. Flipping, shifting and/or tombstoning of SMTcomponents will be referred to herein as SMT component defects or SMTCdefects. The tombstoning effect is considered a common soldering defectin the mounting of SMT components, and is caused by a combination of thesurface tension of the solder, the SMT component's weight, and thesoldering conditions. Another factor contributing to SMTC defects mayinclude a vibration of the conveyor belt transporting the SMT componentduring soldering. SMTC defects having been observed at assembly sitesespecially recently with respect to DSC's whose dimension and weighthave been reduced from 0805 (this terminology means that the componentsthat have a length of 8 mil. and a width of 5 mil.) and 0402 to 0201.Because of the relatively small dimensions and weights of 0402 and 0201components, the intricate balance of the surface tension may be moreeasily disturbed by either the change of the solderability of thecomponents or by the differences of time at which the solder paste ateach end of the component begins to melt.

The prior art has attempted to resolve SMTC defects caused during themounting process by tuning either the solder paste printing process, thesolder reflow process or the solder paste formulation. Tuning the solderpaste printing process typically involves redesigning the printingstencils for the solder pads to change the solder printing parametersfor reflow. Tuning the reflow process on the other hand typicallyinvolves extending the preheating time and the soaking time in order toachieve the desired balance between the surface tension forces on thecomponent's terminals. A slower preheating rate has been shown to reduceSMTC defect rates. Tuning the paste formulation involves employing asolder alloy comprising tin/lead/silver in order to provide a widersolidification range and achieve balance between the surface tension ofboth side of a small leadless component. The expanded solidificationrange lengthens the higher tacky and pasty stage of the solder paste inthe solder deposits, thus balancing a surface tension on the component'sterminals, and in turn reduce the tombstoning frequency.

An alternative measure used in the prior art in order to reduce theoccurrence of SMTC defects contemplates using an adhesive to hold thecapacitor in place during soldering of a pre-mount combination 1 asshown. In such a method, as seen in FIG. 8, where like components arereferred to using like reference numerals with respect to FIGS. 1 a and1 b described above, an adhesive is dispensed on the solder resist 15between the two substrate pads 12 as shown. The adhesive is meant tohold the capacitor in place during soldering in an attempt to reduceSMTC defects. However, disadvantageously, as SMT component sizes shrink,as noted in the paragraph above, use of the adhesive method becomes illsuited to combat SMTC defects to the extent that it among othersrequires an accurate placement of the adhesive and an accuratedispensing of the same, which become more difficult where smallspaces/doses are involved, often requiring a fine tuning of the adhesivedispensing machine. For the reasons stated above, and for other reasonsstated below which will become apparent to those skilled in the art uponreading and understanding the present specification, there is asignificant need in the art for methods for mounting components to asubstrate that offer relatively high density and high qualityinterconnections at a reasonable production cost.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are illustrated by way of example and notby way of limitation in the figures of the accompanying drawings, inwhich the like references indicate similar elements and in which:

FIGS. 1 a is a top plan view of a DSC and substrate combinationaccording to the prior art in which the DSC has shifted onto one of thesubstrate pads;

FIG. 1 b is a side elevational view of a DSC and substrate combinationaccording to the prior art in which the DSC has tombstoned;

FIGS. 2 a and 2 b are side elevational views of a pre-assemblycombination according to an embodiment before and during reflow,respectively;

FIGS. 3 a-3 c are top plan views of three different embodiments of amagnetic layer according to the present invention;

FIG. 4 a is a perspective view of a surface mount component comprising aDSC;

FIG. 4 b is a side elevational view of the DSC of FIG. 4 a being surfacemounted onto a substrate;

FIG. 5 is a side elevational view of a microelectronic assemblyaccording to one embodiment;

FIG. 6 is a schematic representation of a system including amicroelectronic assembly such as the assembly of FIG. 5 according to oneembodiment; and

FIG. 7 is a is a side elevational view of a DSC and substratecombination according to the prior art in which an adhesive is beingused to hold the DSC in place during soldering.

DETAILED DESCRIPTION

A surface mount component including a magnetic layer thereon, a methodof forming the surface mount component, an electronic assembly includingthe surface mount component, and an electronic system including theelectronic assembly are disclosed herein.

Various aspects of the illustrative embodiments will be described usingterms commonly employed by those skilled in the art to convey thesubstance of their work to others skilled in the art. However, it willbe apparent to those skilled in the art that the present invention maybe practiced with only some of the described aspects. For purposes ofexplanation, specific numbers, materials and configurations are setforth in order to provide a thorough understanding of the illustrativeembodiments. However, it will be apparent to one skilled in the art thatthe present invention may be practiced without the specific details. Inother instances, well-known features are omitted or simplified in ordernot to obscure the illustrative embodiments.

Various operations will be described as multiple discrete operations, inturn, in a manner that is most helpful in understanding the presentinvention, however, the order of description should not be construed asto imply that these operations are necessarily order dependent. Inparticular, these operations need not be performed in the order ofpresentation.

The phrase “one embodiment” is used repeatedly. The phrase generallydoes not refer to the same embodiment, however, it may. The terms“comprising”, “having” and “including” are synonymous, unless thecontext dictates otherwise.

Referring to FIGS. 2 a and 2 b, a pre-mount combination 100 is shownprior to and during reflow, respectively. Combination 100 as shownincludes a substrate 110 having bonding pads 112 on a bonding surface113 thereof, and including a solder resist 115 thereon. The shownbonding pads 112 may comprise electroless nickel immersion gold (ENIG)bonding pads. As is well known in the art, an ENIG bonding pad may bemade by providing copper pads using methods well known by those versedin the art. The copper bonding pads may then be put into the propernickel containing bath for a predetermined length of time to deposit aspecific range of nickel thicknesses by electrochemical means. Afterproper rinsing, the bonding pads may then be put into a gold containingelectrochemical bath where the gold atoms spontaneously replace thesurface nickel atoms until the entire nickel surface areas are coveredin gold. A result of the above well know process are bonding pads suchas bonding pads 112, which include a copper layer 112′, a nickel layer112″ thereon, and a gold layer′″ covering the nickel layer. Gold haslong been used in the electronics industry as a metal for contactsurfaces because of its low electrical resistivity and its inertness toattack by corrosive substances. Combination 100 as shown furtherincludes a surface mount component such as DSC 116 having terminals 114.DSC includes a magnetic layer 118 provided on a die-side surface 120thereof.

As seen in FIG. 2 a, the DSC 116 is shown as being in the process ofbeing registered over the substrate 110 such that the terminals 114register with the solder deposits 122 on the bonding pads 112. Beforereflow when the DSC has been registered over the substrate, magneticlayer 118 and nickel layer 112″ (which has ferromagnetic properties) inthe ENIG bonding pads interact to establish a magnetic force MF betweenDSC 116 and substrate 110 that among others advantageously holds the DSCover the substrate to allow the DSC to remain registered over thesubstrate pads before reflow.

As seen in FIG. 2 b, when combination 100 is undergoing reflow, unevensurface tension forces STF between bonding pads 112 on each side of theDSC can produce a torque on the DSC 116 that may be counteracted by amagnetic torque caused by magnetic forces MF acting between magneticlayer 118 and the nickel layer 112″ present in the ENIG bonding pad 112.Such counteraction is effective for substantially preventing SMTCdefects such as flipping, shifting or tombstoning as shown in part inFIGS. 1 a and 1 b. The magnetic layer may be chosen such that MF islarger than STF, increasing a holding force between the DSC andunderlying substrate before and during reflow for bringing about reducedSMTC defects, such as, for example, for 0402 and/or 0201 DSC's.

It is noted that, as used in the instant description, what is meant by“hold” or “holding” refers to holding an SMT component on the substratesuch that, before reflow, the SMT component remains registered on thesubstrate, and, during reflow, the SMT component does not flip, shift ortombstone.

The magnetic layer 118 may be disposed on the DSC, according to oneembodiment, during DSC manufacturing, such as using a conventionalprinting method. According to embodiments, magnetic layer is selected toprovide a magnetic force MF that produces a torque larger than a torqueproduced by uneven surface tension forces STF of the two solder deposits22, while at the same time having minimal impact on the performance ofcircuits on the substrate or on the SMT component. Preferably, amagnetic material is selected having a Courier temperature that isslightly higher than the reflow peak temperature range of the solder toundergo reflow. For example, the Courier temperature of the magneticmaterial chosen may be between about 10 degrees Celsius to about 20degrees Celsius higher than a reflow peak temperature range of thesolder. In such a case, where lead-containing solder is used, the peakreflow temperature range would be between about 210 degrees Celsius andabout 220 degrees Celsius, in which case the Courier temperature rangeacceptable for the purposes of embodiments would be between about 220degrees Celsius and about 240 degrees Celsius. In addition, wherelead-free solder is used, the peak reflow temperature range would bebetween about 240 degrees Celsius and about 250 degrees Celsius, inwhich case the Courier temperature range acceptable for the purposes ofembodiments would be between about 260 degrees Celsius and about 270degrees Celsius. A magnetic material with a Courier temperature belowthe reflow peak temperature range could substantially lose its magneticproperties during reflow, thus disadvantageously leading to an effectivedisappearance of a counteracting magnetic force MF between the SMTcomponent such as DSC 116, and the underlying substrate. Morepreferably, a magnetic material is selected that exhibits a remanenceadapted to have a minimum impact on a performance of circuits within theSMT component or within the substrate. A selection of magnetic materialsbased on remanence and its impact on circuit performance becomesespecially important in the case of circuits having higher frequencies,such as frequencies equal to or above about 2 GHz, as in the case of aCPU. On the other hand, a magnetic material according to embodimentsexhibits a remanence that nevertheless provides the necessarycounteracting force to counteract a torque on the SMT component byunequal surface tension forces between the solder deposits on thesubstrate bonding pads. Examples of magnetic materials that may be usedas part of the magnetic layer according to a preferred embodiment mayinclude any one of nickel or ferronickel alloys. In the case offerronickel alloys, their compositions may be engineered in a well knownmanner to obtain a specific remanence according to application needs.

It would be within the knowledge of one skilled in the art to usetechniques such as simulation, taking into account for example thedimensions, including terminal dimensions, of the SMT component, theweight of the SMT component, and, in addition, the surface tensiontorque on the SMT component from one of the solder deposits, in order toarrive at a magnetic force torque necessary to counterbalance thesolder's torque in order to substantially prevent SMTC defects. Based onthe thus found magnetic force torque, a magnetic layer may be selectedto generate such magnetic torque during reflow. In general, usingguidelines such as those provided in the paragraph above, a magneticmaterial layer may be selected that provides the minimum magnetic forcenecessary to effect the desired counterbalancing of the DSC. Such amagnetic layer may have any thickness and define any pattern based onthe magnetic torque requirements for the specific combination beingevaluated. For example, the magnetic layer according to an embodimentmay have a thickness in the same range as the thickness of the nickellayer in the ENIG pads, that is, between about 1 micron and about 5microns.

It is noted that, as used herein, a “magnetic layer” refers to both acontinuous and a non-continuous layer of magnetic material. Thus,referring by way of example to FIGS. 3 a, 3 b and 3 c, a magnetic layeraccording to embodiments may comprise a continuous layer, as shown intop plan view in FIG. 3 a, or, as shown in top plan view in FIGS. 3 band 3 c, a layer having a discontinuous configuration, such as onedefining a pattern. According to one embodiment, a magnetic layer maycomprise sublayers defining a pattern in top plan view that correspondsto a pattern of the substrate pads. Thus as seen in particular in FIG. 3b, where the SMT terminals define a pattern as shown for example inFIGS. 2 a-2 b, magnetic layer 218 may comprise two sublayers 218′ and218″ that are each configured to be placed on a corresponding one of thesubstrate pads. As seen by way of example in the embodiment of FIG. 3 c,a magnetic layer 218 may comprise sublayers 218′ and 218″ which eachdefine a pattern P as shown. It is noted that FIGS. 3 a-3 c merely showexamples of magnetic layer configurations, and that other configurationsare within the scope of the embodiments of the invention.

With respect to the substrate pads, it is noted that it is not necessaryaccording to embodiments that the pads be ENIG pads. Embodiments of thepresent invention encompass within their scope substrate pads other thanENIG pads as long as the substrate pads include a ferromagnetic materialtherein adapted to cooperate with the magnetic layer as described abovein order to establish a magnetic force to counteract unequal surfacetension forces of the solder deposits.

Example

With respect to the selection of a suitable magnetic layer according toembodiments, the following calculations are provided as an example withrespect to a 0402 DSC referring in particular to the illustrations inFIGS. 4 a and 4 b. Thus, according to one embodiment, DSC 116 may be an0402 DSC, in which case the dimensions and properties of the same are asfollows:

DSC specific gravity:ρ=5.85×10⁻³ g/mm³

DSC volume:V=L×W×H=1×0.5×0.5=0.25 mm³

DSC pad dimension:B×W=0.2×0.5=0.1 mm³

DSC mass:M=ρ×V=1.4625×10⁻³ g

DSC weight:W=ρ×V×g=1.4625×10⁻² NUsing Eutectic SN63/Pb37 as solder, the surface tension would be δ=464mN/mm. As a result, a surface tension force on the DSC's terminal sidewould be F₁=δ×W=232 mN, and the surface tension torque would beT₁=(F₁×H)/cos θ=164×10⁻⁶ NM. On the other hand, the gravity torque wouldbe T₂=(W×L)/(cos θ×2)=9.9×10⁻⁶ Nm. As seen above, T₁>>T₂, which wouldcause the tombstoning of the DSC. Assuming the magnetic force of thefilm is ten times larger than the weight of the capacitor, then F₃=F₄=10W=14.625×10⁻² N, the magnetic torque would be T₃=(F₃×L)/cos θ=206.9×10⁻⁶Nm, and T₄=F₄×0=0. Thus, the total clockwise torque on the DSCTCLT=T₂+T₃+T₄=216.8 u×10⁻⁶ Nm, while the total counterclockwise torqueon the DSC TCOT=T₁=164×10⁻⁶ Nm. Because TCLT/TCOT=216.8/164>1, the DSCwill not tombstone.

Advantageously, embodiments of the present invention provide a simple,cost effective, and operative configuration to hold a SMT component overa substrate before and during reflow. In particular, as compared withconventional methods of minimizing SMTC defects by engineering andmonitoring solder paste formulations, and the associated printing andreflow processes, embodiments of the present invention take advantage offerromagnetic properties of the substrate pads, such as, for example, ofnatural ferromagnetic properties of the nickel layer in the ENIG pad onthe substrate, in order to hold a SMT component over a substrate beforeand during reflow. In addition, advantageously, according to embodimentsof the present invention a stabilizing and holding force between the SMTcomponent and the substrate is a function of a magnetic field of amagnetic layer on the SMT component, the provision of which onto the SMTcomponent would be easier to control when compared with traditionalmethods of minimizing SMTC defects as noted above, and also whencompared with other SCAM process parameters. In addition,advantageously, embodiments of the present invention provide a universalmethod of minimizing SMTC defects without a need to develop separatematerials and/or processes as the size of the SMT components changes.The above is all the more advantageous in light of the miniaturizationtrend surrounding SMT components, such as a transition from 0805DSC's tomuch smaller 0201 DSC's. Additionally, and in particular with respect toDSC's, to the extend the a main function of a DSC is the provision ofstable voltage during a powering on and powering off of a deviceassociated with the DSC as opposed to logic/storage, the provision of amagnetic layer on the DSC according to embodiments would advantageouslysubstantially not affect a functioning of the DSC. Additionally, all ofthe above advantages are possible according to embodiments without thenecessity to make any changes to the mounting/assembly equipment usedfor mounting the SMTC onto the substrate. The above advantages allow astable and high capacitor attach yield and improve a process window forsoldering with substantially no impact to the mounting equipment.

Referring next to FIG. 5, an embodiment of a microelectronic assemblyaccording to the present invention is depicted as assembly 200. As shownin FIG. 5, assembly 200 represents combination 100 of FIGS. 2 a and 2 bafter reflow and subsequent attachment of DSC 116 to substrate 110. Asseen in FIG. 5, assembly 200 shows DSC 116 as having been attached orbonded, that is, electrically and mechanically bonded, to ENIG pads 112of substrate 110 via solidified solder 123. DSC 116 includes a magneticlayer 118 disposed thereon, which, as described above, stabilizes theDSC over the substrate before and during reflow.

Referring now to FIG. 6, there is illustrated one of many possiblesystems 90 in which embodiments of the present invention may be used.The microelectronic assembly 1000 may be similar to the microelectronicassembly 200 depicted above in FIG. 5, respectively. In one embodiment,the electronic assembly 1000 may include a microprocessor. In analternate embodiment, the electronic assembly 1000 may include anapplication specific IC (ASIC). Integrated circuits found in chipsets(e.g., graphics, sound, and control chipsets) may also be packaged inaccordance with embodiments of this invention.

For the embodiment depicted in FIG. 6, the system 90 may also include amain memory 1002, a graphics processor 1004, a mass storage device 1006,and/or an input/output module 1008 coupled to each other by way of a bus1010, as shown. Examples of the memory 1002 include but are not limitedto static random access memory (SRAM) and dynamic random access memory(DRAM). Examples of the mass storage device 1006 include but are notlimited to a hard disk drive, a compact disk drive (CD), a digitalversatile disk drive (DVD), and so forth. Examples of the input/outputmodule 1008 include but are not limited to a keyboard, cursor controlarrangements, a display, a network interface, and so forth. Examples ofthe bus 1010 include but are not limited to a peripheral controlinterface (PCI) bus, and Industry Standard Architecture (ISA) bus, andso forth. In various embodiments, the system 90 may be a wireless mobilephone, a personal digital assistant, a pocket PC, a tablet PC, anotebook PC, a desktop computer, a set-top box, a media-center PC, a DVDplayer, and a server.

Although specific embodiments have been illustrated and described hereinfor purposes of description of the preferred embodiment, it will beappreciated by those of ordinary skill in the art that a wide variety ofalternate and/or equivalent implementations calculated to achieve thesame purposes may be substituted for the specific embodiment shown anddescribed without departing from the scope of the present invention.Those with skill in the art will readily appreciate that the presentinvention may be implemented in a very wide variety of embodiments. Thisapplication is intended to cover any adaptations or variations of theembodiments discussed herein. Therefore, it is manifestly intended thatthis invention be limited only by the claims and the equivalentsthereof.

1. A Microelectronic assembly comprising: a substrate having contactsdisposed on a mounting surface thereof, the contacts including amagnetic material therein; a solder disposed on the contacts; and acomponent bonded to the substrate by way of the solder and including amagnetized magnetic material disposed in a continuous layer acrosssubstantially all of a substrate side thereof, the magnetized magneticmaterial to cooperate with the magnetic material in the contacts toestablish a magnetic force to hold the component on the substrate. 2.The assembly of claim 1 wherein the magnetized magnetic materialcomprises a pattern.
 3. The assembly of claim 1 wherein the componentcomprises a die side capacitor.
 4. The assembly of claim 1 wherein thecontact comprises nickel.
 5. The assembly of claim 1 wherein thecontacts on the substrate comprise electroless nickel/immersion gold(ENIG) pads, and wherein the magnetic material in the contacts comprisenickel.
 6. The assembly of claim 1, wherein the magnetized magneticmaterial comprises a magnetic material having a Curie temperature thatis above a peak reflow temperature range of the solder.
 7. The assemblyof claim 1, wherein the magnetized magnetic material comprises amagnetic material including at least one of a nickel and a ferronickelalloy.
 8. The assembly of claim 1, wherein the magnetized magneticmaterial has a thickness between about 1 micron and about 5 microns. 9.A component bonded to contacts of a substrate by way of a solder, thecomponent including a magnetized magnetic material disposed in acontinuous layer across substantially all of a substrate side thereof,the magnetized magnetic material to cooperate with a magnetic materialin the contacts to establish a magnetic force of a sufficient magnitudeto hold the component on the substrate.
 10. The component of claim 9wherein the component comprises a capacitor.
 11. The component of claim9 wherein the contacts on the substrate comprise electrolessnickel/immersion gold (ENIG) pads, and wherein the magnetic material inthe contacts comprise nickel.
 12. The component of claim 9 wherein themagnetized magnetic material comprises a pattern.
 13. A systemcomprising: a microelectronic assembly including: a substrate havingcontacts disposed on a mounting surface thereof, the contacts includinga magnetic material therein; solder disposed on the contacts; and acomponent bonded to the substrate by way of the solder and including amagnetized magnetic material disposed in a continuous layer acrosssubstantially all of a substrate side thereof, the magnetized magneticmaterial being adapted to cooperate with the magnetic material in thecontacts to establish a magnetic force of sufficient magnitude to holdthe component on the substrate.
 14. The system of claim 13 wherein thecomponent comprises a capacitor.
 15. The system of claim 13 wherein thecontacts on the substrate comprise electroless nickel/immersion gold(ENIG) pads, and wherein the magnetic material in the contacts comprisenickel.